Silicon (Si) normally crystallizes in the diamond cubic (dc) structure, which corresponds to the zinc-blende structure with single atom type. The cubic Si has an emission around 1.1 eV corresponding to an indirect band gap with low intensity radiation. Therefore it is not a material of interest for optoelectronic applications, but it is mainly used as the substrate for microelectronic devices and for solar cells. However, there are at least 13 phases of silicon reported, most of which only form under high pressure conditions and are otherwise unstable. The existence of Si in different crystal structures with different electronic properties could offer the possibility of increased flexibility in the design of future Si-based devices. Among them, hexagonal silicon with lonsdaleite structure, which corresponds to the wurtzite structure with single atom type, and which is also referred to as diamond-hexagonal (dh) Si, Si IV (A) or 2H Si, attracted lot of attention during the last decade. In 2002, Raffy et al. predicted in “Properties of hexagonal polytypes of group-IV elements from first-principles calculations” in Phys. Rev. B. 66, 075201 (2002) that for Si the fundamental energy gap decreases with increasing hexagonality of the polytype.